Summary of significant accounting policies - Textual (Details)		12 Months Ended
	Jul. 19, 2018	Sep. 28, 2019 USD ($) operating_segment	Sep. 29, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 29, 2019 USD ($)
Product Warranty Liability [Line Items]
Stock split, conversion ratio	2
Restricted cash		$ 179,000	$ 190,000
Impairment of long-lived assets		$ 0	0	$ 0
Product warranty term		1 year
Foreign currency remeasurement and transaction gain (losses)		$ (4,035,000)	(941,000)	3,568,000
401(k) plan maximum annual contribution per employee		100.00%
Number of operating segments (in segment) | operating_segment		1
Operating lease liability		$ 60,000,000
Operating lease asset		60,000,000
Goodwill		$ 1,000,000
Accounting Standards Update 2016-02 | Subsequent Event
Product Warranty Liability [Line Items]
Operating lease liability					$ 65,000,000
Operating lease asset					$ 65,000,000
European Union
Product Warranty Liability [Line Items]
Product warranty term		2 years
Other income (expense)
Product Warranty Liability [Line Items]
Foreign currency remeasurement and transaction gain (losses)		$ (8,600,000)	(1,200,000)	3,200,000
Sales and marketing
Product Warranty Liability [Line Items]
Advertising costs		$ 48,800,000	$ 50,200,000	$ 72,200,000
One vendor | Supplier concentration risk
Product Warranty Liability [Line Items]
Concentration percentage		83.00%	98.00%	99.00%